Composing morphs

I'm playing with morphs but having trouble getting it achieved. I have a set of geo coordinates that form a polygon. Over the polygon I want to lay a grid. The polygon I do with


lineMorph
        ^ lineMorph ifNil: [
                lineMorph := (LineMorph
                vertices: self coordinates
                color: Color red
                borderWidth: 1
                borderColor: Color black) ]

The grid I do with

activeGridOn: aMorph
        | min max stepHorizontal stepVertical |
        min := self coordinates min.
        max := self coordinates max.
        stepHorizontal := (max x - min x) / 10.
        stepVertical := (max y - min y) / 10.
       
        min y to: max y by: stepVertical do: [ :y|
                min x to: max x by: (stepHorizontal) do: [ :x | | r |
                        r := Rectangle
                                origin: x@y
                                extent: stepHorizontal@stepVertical.
                        Transcript show: 'drawing rectangle ', r asString; cr.
                        aMorph addMorph:  (BorderedMorph new
                                position: (r topLeft);
                                extent: (r width@r height);
                                color: Color red;
                                borderWidth: 1;
                                borderColor: Color black)
                 ]
        ]

But I cannot see the rectangles because they are too small. It seems the coordinate systems of the line morph and the added morphs are different. Is there a kind of transform need so that the line morph and an added morph can have the same sized coordinate values?

Norbert